A low grade osteosarcoma of the colon

Soft-tissue osteosarcomas are very rare. Only three cases of extraskeletal osteosarcoma have been reported arising in colon and we report the first well differentiated one.

A 58-year-old woman was admitted to our hospital with a history of pain and swelling in lower right abdomen for the last 3 months. Routine laboratory tests and tumor markers (CEA, CA19-9, AFP) were all normal, as well as colonoscopy. CT-scan revealed large partly calcificated mass connected to the ascending colon without significant imbibition of contrast. Because of the size of the tumor, open surgery was indicated. An exploratory laparotomy followed by right hemicolectomy with anastomosis was performed according to oncological principles.

Surgery and pathology confirmed the presence of 13 cm large soft tissue tumor connected to the colon by vascular pedicle. Histopathology showed spindle cells with minimal cellular atypia and focal osteoid production which lead to diagnosis of low grade osteosarcoma. Immunohistochemicaly, neoplastic cells were positive for alpha smooth muscle actin, caldesmon and desmin, but are negative for S100, CD34, CD117, DOG1, BCL-2, epithelial membrane antigen, and beta-catenin.

Postoperative course went uneventful and adjuvant chemotherapy was not recommended. At 6-months follow-up there was no sign of recurrent tumor.